This invention relates to the fabrication of ohmic contacts to n-type Gallium Arsenide. More particularly, this invention relates to such ohmic contacts which are fabricated from an alloy of Aluminum and Germanium.
Contact resistance to n-type Gallium Arsenide is an important process and design parameter for the advancing Gallium Arsenide integrated circuit technology. The Ni-capped, Gold Germanium (12%) eutectic ohmic contact alloy was first introduced by Braslau (N. Braslau, J.B. Gunn and J. L. Staples, "Metal-Semiconductor Contacts for GaAs Bulk Effect Devices." Solid State Electronics, Vol. 10, page 381, 1967) and is now extensively used for Au-Ge-Ni contacts with a variety of techniques and compositions. The eutectic temperature is listed as 356.degree. C. (M. Hansen, Constitution of Binary Alloys, page 97, McGraw Hill Book Company, Inc., New York, 1958) and alloy temperatures up to 450.degree. C. are used to form the ohmic contacts. To complete the ohmic contacts and form the first level of metal interconnects, an overlay of 2500 Angstroms of Au is employed with a thin interspaced layer of Pt or Ti for improved adhesion.
While these contacts have good ohmic properties for device and integrated circuit application, there is room for improvement. For example, Au as a high atomic mass element (197) will tend to absorb relatively more X-ray energy than would a lighter element, leading to deterioration of conductivity. This can have significant consequences in a high energy radiation environment.